Environmental quality assessment is of paramount importance as the world's population approaches eight billion people in the coming decades. Non-polluted soil and water for food production are among the most basic of human necessities. However, industrial development has resulted in widespread pollution and salinization of both soil and water resources (e.g., Pen-Mouratov et al., 2008; Weindorf et al., 2013a; Rengasamy, 2006; Trujillo-Gonzalez et al., 2016). For example, Auchmoody and Walters (1988) documented the impact of brine spills on soils and vegetation in the Allegheny National Forest of Northwestern Pennsylvania, noting that “brine spills and accidental discharges pose serious environmental threats.” Similarly, Vidakovic-Cifrek et al. (2002) note that calcium bromide and calcium chloride are commonly prepared high-density brines used in oil exploration and production. However, they concede that accidental releases of such salts can and do pollute adjacent groundwater and soil.
Soil and water salinity are intrinsically linked as the former relies on slurries or saturated pastes whereby salts precipitated in soil dissolve into solution and are measured in the aqueous phase. Traditional analysis of water salinity has commonly used electrical conductance. For decades, the Solubridge (a rudimentary, analog salinity bridge) was the preferred technology (Salinity Laboratory Staff, 1954), whereas more recently, the use of digital conductivity meters has become commonplace. Conductivity meters rely on the fact that dissolved cations and anions in aqueous solution effectively bolster the transmission of electric current (Rhoades, 1996). Thus, the more dissolved salt present in aqueous samples, the greater the electrical conductivity (EC). However, salinity meters only provide information on the total dissolved solutes found in water; they do not identify the types of cations/anions which contribute to the salinity of the sample. Elemental identification/quantification has been reliant first upon flame photometry, then atomic absorption spectroscopy (MS) (Wright and Stuczynski, 1996), and finally inductively coupled plasma atomic emission spectroscopy (ICP-AES) (Soltanpour et al. 1996). Colorimetry and other wet chemistry techniques may also be used for the identification and qualitative estimation of certain elements (e.g., Kuo, 1996).
Applied to soil and environmental science, several methods have been established for portable X-ray fluorescence (PXRF) spectrometry use in soil, sediment, and other matrices; among them Method 6200 (US-EPA, 2007), a recently established method by the Soil Survey Staff (2014), and a method by the Soil Science Society of America (Weindorf and Chakraborty, 2016). Specific applications of PXRF for soil and environmental quality analysis include quantification of cation exchange capacity (Sharma et al. 2015), soil reaction (pH) (Sharma et al. 2014), enhanced soil horizonation (Weindorf et al. 2012), plant essential nutrients (McLaren et al. 2012), and the spatial variability of pollutants (Paulette et al. 2015; Clark and Knudsen, 2014). However, three studies in particular indicate the possible utility of PXRF for elemental determination in water samples. Weindorf et al. (2013b) used PXRF for the determination of gypsum (CaSO4.2H2O) with Ca and Sas proxy elements for gypsum content. Using simple linear regression, PXRF-determined S and Ca predicted total gypsum content, as determined by thermogravimetry, with R2 of 0.91 and 0.88, respectively. Similarly, Swanhart et al. (2014) used Cl as a proxy for saturated paste soil salinity and found a calibration R2 of 0.83, with an even stronger R2 of 0.90 using multiple elements and multiple linear regression. As both of these represent salts present in soil, there is reason to believe that PXRF may be effective in determining water salinity using single or multiple elements as proxies (e.g., Cl, Ca, S, K).
Moreover, water often has various elements (e.g., Ca, Mg, Cl, F) dissolved in it which pose little concern to organisms so long as concentrations are relatively low. When concentrations of dissolved salts become too high, the water is often termed brackish or salt-water. However, high concentrations of dissolved metals in water can pose a serious public health risk as such substances are commonly non-detectable without laboratory analysis. In some instances, the metals dissolved in water come from natural geologic sources. Globally, saline and sodic soils constitute 397.1 and 434.3 million ha (UN-FAO, 2016) while polluted soils are found at hundreds of thousands of sites worldwide. For example in Europe, heavy metals from smelting activities account for 34.8% of polluted soils (IASS, 2012; Paulette et al., 2015; Weindorf et al., 2013). This pollution, in turn, represents a significant threat to human health (Brevik and Sauer, 2015). More specifically, Berg et al. (2001) found As levels in raw groundwater used to supply Hanoi treatment plants often surpassed World Health Organization (WHO) limits of 10 μg L−1, the origins of which stem from the Red River Basin. Nordstrom (2002) detailed an extensive list of countries with As-laden groundwater including Bangladesh, India, Argentina, Chile, Germany, Hungary, Romania, USA, and many others whereby As is naturally occurring from geologic sources. In other cases the metals stem from industrial pollution, mining, or waste migration into surface or subsurface waters used for drinking. For example, smelting operations in Eastern Europe left widespread metal pollution across surface soils (Paulette et al., 2015; Weindorf et al., 2013). Similarly, Razo et al. (2004) found surface water storage ponds in Villa de la Paz-Matehuala contained As levels more than five times the Mexican drinking water standard. More recently in the United States, the city of Flint, Mich. (pop. ˜100,000) experienced a public health crisis when the city's water supply became contaminated with Pb (Hanna-Attisha et al., 2016). In 2015, the Gold King Mine spill in Colorado, USA released hundreds of thousands of gallons of acid mine drainage waste into the Animas River; a source of irrigation water for the farming communities of Farmington, N.M. and the Navajo Nation (Rodriguez-Freire et al., 2016). Some studies have also established linkages between metal content in water and other chemical factors such as pH (Muhammad et al., 2011) and conductivity (Kar et al., 2008).
The WHO (2008) has established numerous chemical limits for various elements in drinking water in order to assure human health and safety. For example, the WHO drinking water guidelines for Pb, Zn, and Cu are 10 μg L−1, 3 mg L−1, and 2 mg L−1, respectively. In determining water quality, the WHO reviews a number of analytical methods, among them flame atomic absorption spectrometry (FAAS), atomic absorption spectrophotometry (AAS), electrothermal atomic absorption spectrometry (EAAS), and inductively coupled plasma mass spectroscopy (ICP-MS). By comparison, only two field methods are noted: colorimetry and absorptiometry. Thus, fewer field methods are available and with less analytical precision and accuracy, relative to laboratory approaches.
Recently, PXRF has rapidly developed as a field-portable instrument capable of producing multi-elemental data with limited sample preparation. Accuracy of PXRF generally improves with increasing atomic number, as elemental quantification is tied to electron shells which become increasingly dense as atomic number increases. The Royal Society of Chemistry (2009) provides a succinct overview of the technology whereby a miniature X-ray tube dissipating a few watts is used to excite elements, thereby causing them to generate secondary fluorescence X-rays with characteristic energies for each element. Elemental abundance is quantified via silicon drift detector (SDD), which provides “higher resolution with little degradation in spectrum quality (e.g., count rate-dependent peak broadening or drift)” relative to silicon PIN detectors (Royal Society of Chemistry, 2009). Matrix interference is caused by inter-elemental effects whereby emission line overlap and other background variation must be resolved through signal processing (Peinado et al., 2010). While PXRF is theoretically capable of determining many elements, the excitation of low atomic number elements (e.g., <K) is often problematic given fluorescence attenuation in air. Helium purge or vacuum attachments can overcome some of these limitations, but PXRF determination on low atomic number elements remains problematic (Weindorf et al., 2014). Those limitations notwithstanding, numerous methods now exist for PXRF evaluation of elements in soil and sediment (US-EPA, 2007; Soil Survey Staff, 2014). A litany of studies have established its use for soil (e.g., McLaren et al., 2012a; Zhu et al., 2011; Chakraborty et al., 2017a) and vegetal analysis (e.g., McGladdery et al., 2018; McLaren et al., 2012b; Reidinger et al., 2012). However, the evaluation of liquids by PXRF is comparatively sparse. An early study by Eksperiandova et al. (2002) evaluated wastewater by PXRF using agar and gelatin as a holding matrix for polluted waters. They obtained reasonably low relative standard deviations (up to 0.08%) for several metals at low concentrations (<400 mg L−1). Pearson et al. (2017) extended the use of PXRF by directly determining water salinity based upon elemental determinations of brine waters in a hooded test stand. Using piecewise linear regression of PXRF sensed Cl, they obtained R2 values of 0.77 (RMSE 0.95 μS cm−1) relative to electrical conductance. Further unpublished data by Pearson et al. investigated the utility of PXRF to quantify metals in standard ICP calibration solutions. Results showed the potential for multi-elemental determination with accuracy of □±10% relative to certified reference values.
As a result, there is a need for a portable apparatus and method for liquid chemical characterization, namely salinity and metal content, using PXRF.